(a) Field of the Invention
The present invention relates to an optical scope, and more particularly, to an optical scope in which a lens having negative power is disposed on an image formation surface of an objective lens, thereby increasing eye-relief.
(b) Description of the Related Art
In firearms, a sighting means may be coupled to a top portion of the firearm so as to accurately aim an external target. In a particular case of a rifle among the firearms, aiming is achieved by aligning a line of sight between a sight and a bead, in which speed showing how quickly the aiming is achieved to fire an aimed shot and accuracy showing how accurately the aimed shot hits the target are very important.
That is, an aimed-shooting method requires complicated procedures and time to acquire and ascertain a target, arrange the line of sight, aim at the target, etc. Also, because the sight and the bead themselves are very small, eyes are turned upon the sight and the bead rather than the target or a frontward situation and therefore a field of view becomes narrow if excessive attention is paid to the alignment for the line of sight in order to accurately align the sight and the bead.
Accordingly, an optical scope has been proposed to solve the above cumbersome alignment for the line of sight and improve the accuracy a little more.
The optical scope employs a magnifying-power optical system, which includes an objective lens and an objective lens reticle (i.e., the light of sight), to magnify a target, and is thus excellent in discerning the target, thereby enabling steady aiming through the reticle placed inside the scope.
Such an optical scope is broadly classified into an erecting prism type and a relay lens type. FIG. 1 shows structures of these two types.
First, referring to (a) of FIG. 1, the erecting prism type optical scope includes an objective lens 12, a prism optical system 14, a reticle 13, an eyepiece lens 11, etc. If an image of an external object from the objective lens 12 is formed at a position of the reticle 13, both the image and the reticle 13 are magnified and viewed through the eyepiece lens 11, which is the principle of a telescope or scope. At this time, if the image from the objective lens 12 is directly formed at the position of the reticle 13, the image is viewed as it is inverted. Thus, an erecting prism 14 is provided between the objective lens 12 and the reticle 13 to erect the image viewed through the eyepiece lens 11 by inverting the inverted image again. As the kind of erecting prism 14, there are an Abbe prism 14a (also called a Koenig prism or a Brashear-Hastings prism) as shown in FIG. 2, a roofed Pechan prism 14b as shown in FIG. 3, etc.
Referring to a cross-section of the erecting prism shown in FIG. 4, which is a roofed Pechan prism 14b having a face length of 17.5 mm, a roof having a right-angled prism structure is formed on a top portion of an above prism, thereby serving to invert the left and right images. A total calculated geometrical path is 77.49 mm if an air space between two prisms is 0.70 mm. The shortest distance on an optical axis of this roofed Pechan prism is 21.12 mm, and a expansion amount of the optical path length (OPL) due to the refractive index (1.5168@d-line) of the prism glass BK7 is
                    (                  77.49          -          0.70                )            ×                        (                      n            -            1                    )                n              =          26.16      ⁢                          ⁢      mm        ,
so that the geometrical effect on reducing a body tube is 56.37 mm, and a calculated effect on reducing the optical path is 30.21 mm.
In general, the erecting prism type is advantageous to shorten the body tube, but difficult to manufacture the prism.
Meanwhile, FIG. 1 illustrates one sheet of the objective lens 12 and one sheet of the eyepiece lens 11, but many sheets of them are generally provided in practice to remove aberration or the like
Referring to (b) of FIG. 1, the relay lens type optical scope includes an objective lens 12, a field lens 16, a reticle 13, a relay lens 15, an eyepiece lens 11, etc. If an image of an external object from the objective lens 12 is formed at a position of the reticle 13, both the image and the mark of the reticle 13 are formed again in front of the eyepiece lens 11 and magnified and viewed through the eyepiece lens 11, which is the principle of a relay lens type telescope. That is, if the image from the objective lens 12 is directly formed on the reticle 13, the image is generally viewed as it is inverted. This inverted image is formed once gain by the relay lens 15 and thus inverted again, so that the image in front of the eyepiece lens 11 can be erect. Then, this erect image is magnified and viewed through the eyepiece lens 11.
Generally, the optical scope includes the objective lens 12, the eyepiece lens 11, the reticle 13, and the relay lens 15 or the erecting prism 14 for erecting the image. In addition, the field lens 16 having the positive power may be interposed between the reticle 13 and the eyepiece lens 11 so as to broaden the field of view.
In designing the eyepiece lens of the scope, technology that the field of view through the eyepiece lens 11 is broadened by placing the field lens 16, which has the positive power and interposed between the reticle 13 and the eyepiece lens 11, near the reticle 13 has been applied to the scope such as the existing telescope or the like. As the field of view is broadened, eye-relief (refer to ‘D’ in FIG. 5) generally becomes short. Nevertheless, since there is no need for increasing the eye-relief in the existing telescope, this technology has been mostly used.
As shown in FIG. 5, it is possible to reduce more damage due to recoil of shooting in a firearm G and to quickly acquire motion of a target and its surroundings as eyes of a shooter become distant from the scope 10. However, a conventional scope 10 employs the objective lens 12, the eyepiece lens 11, the reticle 13, the relay lens 15 or the erecting prism 14 for erecting an image, and the field lens 16 having the positive power, so that a shooter's eyes gazing upon the scope 10 cannot be sufficiently distant from the scope 10.